Potential-Assisted Deposition of Alkanethiols on Au:  Controlled Preparation of Single- and Mixed-Component SAMs

Abstract

The commonly used method of preparing RS/Au self-assembled monolayers (SAMs) involves a passive incubation process in a nonabsorbing solvent. Preparation of mixed-component SAMs is particularly problematic under these conditions. The time course of the open circuit potential in the passive adsorption experiment suggests that control of the deposition potential could lead to a faster and ultimately more complete SAM formation process. This is shown to be the case, as both C16 RS/Au SAMs and mixed C16/HOOCC15S/Au SAMs are shown to be readily prepared in approximately 15 min from 5 mM thiol solutions at potentials ranging from 200 to 600 mV (vs Ag/AgCl). The blocking properties of the resulting C16 SAMs are excellent. This technique provides access to mixed-composition SAMs otherwise inaccessible using deposition under open circuit conditions. Although RSH/Au self-assembled monolayers (SAMs) have been the subject of several hundred scientific reports, the mechanism by which they form is relatively poorly understood and characterized. It is also widely acknowledged by practitioners in the field that it is difficult to obtain SAMs with identical function (i.e. electron-transfer blocking capability 1a ) or structure (i.e. determined by diffraction 1b ) although ostensibly identical preparation conditions may have been used. Although it is often stated that SAMs offer a chemist the ability to "engineer" a surface with "tuned" properties, the preparation of both single-component and multicomponent SAMs in a predictable manner is problematic. 2 We are very interested in (i) obtaining high-quality, extensively blocked (to aqueous redox couples) SAMs formed from n-alkanethiols and (ii) obtaining binary and higher order SAMs whose composition is both predictable and reproducible. To this end we have found that the chemisorption of the alkanethiol can be controlled electrochemically. A modest anodic potential applied to the gold electrode during RSH adsorption very quickly leads to high-quality SAMs with excellent performance characteristics. This potential deposition process also allows one to precisely control the ratio of components in a model binary SAM made up of C 16 SH and HSC 15 CO 2 H.Alkanethiol monolayer systems have been most commonly prepared either by incubation of a gold substrate in a thiol-containing solution or by deposition from the gas phase. 2 Full, close-packed coverage is, however, problematic from the gas phase. 2 In neither method is the potential of the gold substrate fixed, and in the solution incubation experiment the complexities arising from a Nernstian redox pinning are not generally acknowledged. Although the self-assembly process is simple, it is often both time-consuming and irreproducible. 1a,b In the solution-based process, the surface coverage increases with increased incubation time 3-5 and fully covered, highly ordered monolayers may require days or even weeks to prepare. 3-6 Furthermore, it is difficult or impossible to prepare binary (or higher order) SAMs with predictable composition ...

Electrochemical nucleic acid-based biosensors: Concepts, terms, and methodology (IUPAC Technical Report)

“…The dsDNA immobilization on gold can be controlled by electric potential. For instance, faster formation of a more compact layer of the thiolated ssDNA was achieved under application of low positive potential (+0.2 V vs. SCE) [57].…”

The Thioacetate-Functionalized Self-Assembled Monolayers on Au: Toward High-Performance Ion-Selective Electrode for Ag+

“…[10][11][12][13][14] Self-assembled monolayers (SAMs) are formed at a solidliquid interface by molecules consisted of three units: a headgroup that binds to the substrate, a tailgroup that constitutes the outer surface of the film, and a backbone, usually a long chain, that connects the headgroup and the tailgroup. While pure alkylthiols are known to form SAMs that exhibit order on a large scale, [15][16][17][18] -conjugated thiols and their derivatives seem to be a better choice for applications in organic-and single-molecule electronics because of their intrinsic semiconducting properties. Relatively recently, Piotrowski et al 19 reported the synthesis and electronic properties of thioacetate-functionalized fullerene, Park et al 20 studied the formation of octylthioacetates SAMs on gold in the vapor phase.…”

The spatial and sequential immobilisation of cytochrome c at adjacent electrodes

“…[1][2][3][4] One of the challenges in the development of more efficient biofuel cells is to perform multistep oxidation of a fuel such as glucose to enhance the power output of the cell. 5 Enzymatic biofuel cells in particular face a 20 significant limitation in this regard due to the specificity of enzymes, limiting their use to single substrates. Enzyme cascades can alleviate this limitation by enabling the sequential and more complete oxidation of the fuel.…”

“…Application of an appropriate potential can also be used for the rapid formation and desorption of SAM layers, with electrode modification occurring on time scales of 70 minutes as opposed to the longer time scales (hours) used for solution deposition. [17][18][19][20][21] Typically, SAM formation is performed in nonaqueous solutions, due to the limited solubility of thiols in aqueous buffer. An example of the electrochemically promoted adsorption and desorption of an alkanethiol is that of 75 dodecanethiol in a solution containing 0.5 M KOH in aqueous ethanol.…”

“…A layer of 1-hexadecanethiol (in 50% aqueous ethanol (50% v/v) was deposited electrochemically on two gold disc electrodes (A and B) to create two separate passivated surfaces (step 1). 20 Electrochemical adsorption of SAMs can be achieved using a range of conditions, e.g. the adsorption of octadecanethiol was accomplished by using an applied potential of 0.5 V for 5 min.…”